Yes, it is a fertilizer issue; but too little, not too much. Note the small yellow fruits, which are being aborted because the tree cannot support them.

A tree this size with 20-30 fruits will suck the nutrients out of older leaves to support new leaves, fruits, etc.

An inground Meyer should be fertilized with a good citrus fertilizer 3-4 times per year and at 1.5 times the label recommendation.

I have found it nearly impossible to over fertilize a garden Meyer, which has to compete with grass and neighboring plants for food and water.

Thank you! It seemed odd that just as the fruit was appearing my leaves started to turn yellow, so I thought it might be related.

Any idea of it is the same issue with my dwarf mandarin (photo attached)? The mandarin has only been in the ground for 2 months and there is no fruit given the season. The nursery told me it was ok to fertilize a young tree.

Christina

That looks to be a chemical burn of some sort; probably was sprayed with something when the weather was too hot.
Aside from that, I don't know much about mandarins.

Based upon the initial description of the lemon and how it looks a year later you have a green thumb and local conditions are good.

I disagree with the fertilizer diagnosis on a tree only in the ground 1.5 years and would suspect it is much more likely due to the alkaline nature of your soils in San Diego. The iron chlorosis on your tree may be able to be treated with fertilizer fortified with an iron supplement but would better be treated by applying soil sulfur per mfg. directions so your tree can take up the nutrients in the soil rather than dumping some nutrients onto the soil and hoping a fraction can be taken up by the struggling tree. It will take many weeks before the pH starts to drop, but frankly most citrus this time of year exhibits some yellowing in the SW.

While an iron chelate like Fe-EDDHA can be used for immediate relief that tree of your does not appear to be suffering badly. Additionally a foliar application of iron on a small area of yellowed leaves can confirm the deficiency overnight.

As for the mandarin I was initially thinking spider mites but looking over your entire photo convinces me it is it more likely a young tree with a western or southern exposure and/or the same alkaline soil.

Here is a link that might be useful: soil sulfur in the desert

Thank you for the comments. I'm going to add more fertilizer to start with. Since this change in leaf color happened as soon as the fruit started to develop, I thought it might be related to nutrition, but was concerned about over-fertilizing. Last year I cut off all of the early fruit to allow the tree more time to mature and this year I'm allowing it to fruit. If there's no improvement within a few weeks I will consider a soil analysis. I'm stumped on my mandarin and may create a separate post for that. Your time and comments were much appreciated.

Christina